Fuel vapor leak detection system

ABSTRACT

Differential flow sensing through respective flow path portions of a system for detecting leakage from an evaporative emission control system. A series flow path comprises a first flow sensor for sensing the entire output flow of an impeller pump used to positively pressurized the system under test, a second sensor in a portion of the series flow path that is downstream of the first flow sensor, and a calibrated orifice downstream of the second flow sensor. A branch circuit to the evaporative emission space under leakage test tees into the aforementioned series circuit between the first flow sensor and the second flow sensor. A differential amplifier receives signal from the respective sensors. Any difference in sensed flows, once the pressure has been brought to a predetermined pressure, is output by the differential amplifier to signify leakage in an amount correlated with the sensed difference in the flows.

REFERENCE TO A RELATED APPLICATION

In certain respects this is an improvement on the inventors' priorapplication entitled "Fuel Vapor Leak Detection System", Ser. No.08/205,983, filed Mar. 4, 1994, now U.S. Pat. No. 5,390,645, issued Feb.21, 1995.

FIELD OF THE INVENTION

This invention relates to an on-board system for detecting fuel vaporleakage from an evaporative emission control system of an automotivevehicle.

BACKGROUND AND SUMMARY OF THE INVENTION

The referenced patent application discloses a leak detection system thatemploys a differential flow sensing principle for detecting fuel vaporleakage from the evaporative emission system of an automotive vehicleduring a leakage test that involves closing the canister purge solenoid(CPS) valve and then positively pressurizing that portion of theevaporative emission system that is upstream of the CPS valve relativeto the engine. One of the significant advantages of that leak detectionsystem is that it less complicated, and hence more economical andreliable, than prior systems not using the differential flow sensingprinciple.

While the present invention also utilizes a differential flow sensingprinciple, it does so in a new and unique way that is less influenced bycertain variables, such as ambient temperature, pressure, enginemanifold vacuum, or supply voltage. Accordingly, a leak detection systemembodying principles of the present invention possesses potential forimproved accuracy and for avoiding false readings that might otherwisearise on account of such variables. More specifically, the inventiveleak detection system is capable of canceling out effects of threeexternal variables by comparing the leakage flow with a known calibratedreference leakage. The inventive leak detection system also eliminatesthe need for a relatively expensive pressure transducer used in priornon-differential flow sensing systems to detect pressure decay. It alsodoes not require a canister vent valve used in such prior systems.

Another aspect of the invention relates to the physical integration ofcertain components of the leak detection system with a fuel vaporcollection canister. This aspect provides greater spatial economy (i.e.more compactness) so that when the integrated canister/system isinstalled in an automotive vehicle, a smaller amount of space is takenup. This savings in space is especially important to many automobilemanufacturers, who must build vehicles that comply with both applicableevaporative emission laws and regulations and applicable fuel economylaws and regulations. The integration possesses the further advantage ofrequiring fewer connections of components in the automotive assemblyplant, and this affords the opportunity for installation cost savingswhile at the same time an opportunity for increased reliability of theinstallation.

Still another advantage of the invention is that a fuel vapor leakdetection test can be performed either when the engine is running or notrunning.

The foregoing, along with further features, advantages, and benefits ofthe invention, will be seen in the ensuing description and claims, whichare accompanied by drawings. The drawings disclose a presently preferredembodiment of the invention according to the best mode contemplated atthis time for carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a fuel vapor leak detection systemembodying principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an exemplary application of the inventive principles to anautomotive vehicle that is powered by an internal combustion engine 10that is controlled by an electronic engine control 12. The vehiclecomprises a fuel tank 14 for storing a volatile fuel such as gasolinethat is vaporized and combusted in the engine's combustion chamberspace.

The associated evaporative emission control system is designatedgenerally by the reference numeral 16 and comprises: the usual canisterpurge solenoid (CPS) valve 18 having inlet and outlet ports 18i and 18orespectively; and a vapor collection canister (sometimes called acharcoal canister) 20 having a tank port 20t, a vent port 20v, and purgeport 20p.

The leak detection system embodying principles of the invention is showngenerally at 22 and comprises a pressure source 24, in the form of adevice (preferably electrically operated, but alternatively operable bya mechanical input) that, when operated during a test, is capable ofpressurizing the evaporative emission space under test, and that, whennot being operated, provides venting of the canister vent port toatmosphere. An example of such a device 24 is an electric motor drivenimpeller pump having an inlet 24i and an outlet 24o.

Leak detection system 22 further comprises a series flow path comprisinga first flow sensor 26, a second flow sensor 28, and a calibratedorifice 30 in that order. Flow sensors 26, 28, in cooperation with adifferential amplifier 32, form a differential flow sensor for sensingthe difference in flow through the respective flow sensors. An exemplarydevice that can be used for each flow sensor is a thermistor, with thetwo thermistors being connected in electric circuitry that is connectedto respective inputs 32a, 32b of differential amplifier 32. Amplifier 32functions to take the difference between the two flows through therespective flow meters and provide at its output 32c, an output signalthat is indicative of the flow differential. This output signal isdelivered to engine control 12, which is a conventional processorprogrammed to conduct a leak detection test, in addition to controllingother engine functions, including possibly other types of diagnostictests.

Canister tank port 20t is coupled with headspace of tank 14 via aconduit 34; purge port 20p is coupled with inlet port 18i of CPS valve18 via a conduit 36; and vent port 20v is coupled with leak detectionsystem 22 via a conduit 38 that forms a branch circuit that tees intothe aforementioned series flow path at a location between the two flowsensors. Outlet port 18o of CPS valve 18 is coupled to the intakemanifold vacuum that is produced by engine 10 when running.

When no leakage test is being performed, CPS valve 18 is operated byengine control 12 to periodically purge the vapor stored in canister 20to the engine. The exact scheduling of such purging is controlled by thevehicle manufacturer's requirements.

When a leak detection test is to be conducted on the evaporativeemission control system, CPS valve 18 is operated closed by control 12,and pressure source 24 is operated to beginning drawing outside airthrough a filter 40 and begin pressurizing that portion of theevaporative emission space comprising tank 14, conduit 34, canister 20,conduit 38, and conduit 36 up to the closed CPS valve. Naturally thevehicle tank filler cap must be in place to close the tank filler tube.

The series flow path formed by the two sensors 26, 28 and calibratedorifice 30 conducts all or a portion of the flow from pressure source 24back to atmosphere through filter 40. Once pressure source 24 has beenoperated long enough to build the pressure inside the evaporativeemission space being tested to a predetermined positive pressurerelative to atmosphere, a leakless evaporative emission space, asdefined above, will result in no flow through the branch conduit 38,while the entire flow from device 24 will pass through the series flowpath composed of the two flow sensors and calibrated orifice. Under thiscondition, equal signals will be input to differential amplifier 32 sothat a signal indicative of no leakage will be given by the amplifierfor logging in memory of control 12.

On the other hand, if there is some leakage from the defined evaporativeemission space once the pressure source has been operated long enough tobuild the pressure to a desired predetermined positive pressure relativeto atmosphere, then some portion of the flow out of device 24 will flowthrough branch conduit 38 after having passed through sensor 26 inaccompaniment of the remainder that proceeds to pass through sensor 28and orifice 30. Since the flow through sensor 28 and orifice 30 will nowbe less than the flow through sensor 26, a difference in input signalswill be detected by differential amplifier 32 and a correspondingleakage signal will be given to control 12 for logging in its memory. Ifthe signal is larger than a predefined value, it indicates the presenceof unacceptable leakage; a lesser signal indicates the absence ofunacceptable leakage.

The broken line that bounds leak detection system 22 in FIG. 1 isintended to show that the various components within its confines canform an assembly that directly mounts on the top of canister 20. Such aphysical integration of components provides more compactness for acanister/leak detection system and requires fewer connections ofcomponents in an automotive assembly plant, affording the opportunityfor installation cost savings while at the same time an opportunity forincreased installation reliability.

While a presently preferred embodiment of the invention has beenillustrated and described, it is to be appreciated that the principlesmay be practiced in other equivalent ways within the scope of thefollowing claims.

What is claimed is:
 1. In an engine-powered automotive vehicleevaporative emission control system wherein a fuel tank for storingvolatile fuel that is combusted in combustion chamber space of theengine is operatively associated with a fuel vapor collection canisterthat collects volatized fuel from the tank and a canister purge valvethat is periodically operated to purge collected vapor from the canisterto the engine, and leak detection system is operatively associated withthe evaporative emission control system for detecting leakage from thatportion of the evaporative emission control system which is upstream ofan inlet of the canister purge valve relative to the engine, theimprovement in said leak detection system which comprises:pressurizingmeans having an inlet at which a source of gaseous medium is availableto be pressurized by said pressurizing means and an outlet at whichpositively pressurized medium is made available for delivery to saidportion of the evaporative emission control system; a series flow pathfrom said outlet of said pressurizing means back to said source ofgaseous medium, said series flow path comprising a first flow sensor forsensing the entire flow from said outlet of said pressurizing means, asecond flow sensor for sensing flow through a portion of said seriesflow path that is downstream of said first low sensor and a calibratedorifice means also disposed in said portion of said series flow paththat is downstream of said first flow sensor; a branch flow path thatbranches to said portion of said evaporative emission control systemfrom said series flow path at a location in said series flow path thatis between said first flow sensor and said second flow sensor; and meansfor sensing differential between flow sensed by said first flow sensorand flow sensed by said second flow sensor.
 2. The improvement set forthin claim 1 in which said pressurizing means comprises an electricallyoperated impeller pump.
 3. The improvement set forth in claim 2 in whichsaid electrically operated impeller pump is physically mounted on saidcanister.
 4. The improvement set forth in claim 2 in which saidelectrically operated impeller pump, when not being operated, serves toprovide a vent from said evaporative emission control system to saidsource of gaseous medium.
 5. The improvement set forth in claim 2 inwhich said electrically operated impeller pump, when not being operated,serves to provide a vent from said first flow sensor to said source ofgaseous medium.
 6. The improvement set forth in claim 5 including afilter through which said electrically operated impeller pump, when notbeing operated, serves to provide a vent from said evaporative emissioncontrol system by way of said first flow sensor to said source ofgaseous medium.
 7. The improvement set forth in claim 6 in which saidsource of gaseous medium is atmospheric air.
 8. The improvement setforth in claim 1 including a filter through which said pressurizingmeans, when not being operated, serves to provide a vent from saidevaporative emission control system to said source of gaseous medium. 9.The improvement set forth in claim 1 in which said means for sensingdifferential between flow sensed by said first flow sensor and flowsensed by said second flow sensor comprises a differential amplifier.10. In an engine-powered automotive vehicle evaporative emission controlsystem wherein a fuel tank for storing volatile fuel that is combustedin combustion chamber space of the engine is operatively associated witha fuel vapor collection canister that collects volatized fuel from thetank and a canister purge valve that is periodically operated to purgecollected vapor from the canister to the engine, and leak detectionsystem is operatively associated with the evaporative emission controlsystem for detecting leakage from that portion of the evaporativeemission control system which is upstream of an inlet of the canisterpurge valve relative to the engine, the improvement in said leakdetection system which comprises:means for creating a certain pressurein said portion of the evaporative emission control system for enablinga leakage test to proceed; a series flow path extending between an inletport and an outlet port of said means that are exposed to a commonpressure, said series flow path comprising a first flow sensor forsensing the entire flow from one of said ports, a second flow sensor forsensing flow through a portion of said series flow path that isdownstream of said first flow sensor and a calibrated orifice means alsodisposed in said portion of said series flow path that is downstream ofsaid first flow sensor; a branch flow path that branches to said portionof said evaporative emission control system from said series flow pathat a location in said series flow path that is between said first flowsensor and said second flow sensor; and means for sensing differentialbetween flow sensed by said first flow sensor and flow sensed by saidsecond flow sensor.
 11. The improvement set forth in claim 10 in whichsaid means for creating a certain pressure in said portion of theevaporative emission control system for enabling a leakage test toproceed comprises a pump for developing positive pressure in saidportion of the evaporative emission control system.